Many types of fluids have been used in well bores particularly in connection with the drilling of oil and gas wells. The selection of an oil-based well bore fluid involves a careful balance of the both the good and bad characteristics of such fluids in a particular application. The primary benefits of selecting an oil-based drilling fluid include: superior hole stability, especially in shale formations; formation of a thinner filter cake than the filter cake achieved with a water based mud; excellent lubrication of the drilling string and downhole tools; penetration of salt beds without sloughing or enlargement of the hole as well as other benefits that should be known to one of skill in the art. An especially beneficial property of oil-based muds is their excellent lubrication qualities. These lubrication properties permit the drilling of wells having a significant vertical deviation, as is typical of off-shore or deep water drilling operations or when a horizontal well is desired. In such highly deviated holes, torque and drag on the drill string are a significant problem because the drill pipe lies against the low side of the hole, and the risk of pipe sticking is high when water based muds are used. In contrast oil-based muds provide a thin, slick filter cake which helps to prevent pipe sticking and thus the use of the oil-based mud can be justified.
Despite the many benefits of utilizing oil-based muds, they have disadvantages. In general, the use of oil based drilling fluids and muds has high initial and operational costs. These costs can be significant depending on the depth of the hole to be drilled. However, often the higher costs can be justified if the oil based drilling fluid prevents the caving in or hole enlargement which can greatly increase drilling time and costs.
In general, drilling fluids should be pumpable under pressure down through strings of drilling pipe, then through and around the drilling bit head deep in the earth, and then returned back to the earth surface through an annulus between the outside of the drill stem and the hole wall or casing. Beyond providing drilling lubrication and efficiency, and retarding wear, drilling fluids should suspend and transport solid particles to the surface for screening out and disposal. In addition, the fluids should be capable of suspending additive weighting agents (to increase specific gravity of the mud), generally finely ground barites (barium sulfate ore), and transport clay and other substances capable of adhering to and coating the borehold surface.
Drilling fluids are generally characterized as thixotropic fluid systems. That is they exhibit low viscosity when sheared, such as when in circulation (as occurs during pumping or contact with the moving drilling bit. However, when the shearing action is halted, the fluid should be capable of suspending the solids it contains to prevent gravity separation. In addition, when the drilling fluid is under shear conditions and a free-flowing near-liquid, it must retain a sufficiently high enough viscosity to carry all unwanted particulate matter from the bottom of the well bore to the surface. The drilling fluid formulation should also allow the cuttings and other unwanted particulate material to be removed or otherwise settle out from the liquid fraction.
Thus a search has been going on for many years for an improved additive for modifying and controlling the solids tolerance properties of drilling fluids that will provide fluids which are efficient, easily handled, and readily dispersible, and usable under high solids contamination and other broad range of conditions.
The present invention is generally directed to a fluid having utility in subterranean wells having improved solids tolerance capabilities. One such illustrative fluid includes an oleaginous fluid; and a solids tolerance agent having the formula: 
in which R is a C6 to C20 aliphatic group and Rxe2x80x2 is a C2 to C6 aliphatic group and x has a value from about 1 to about 10. In one preferred embodiment of the above fluid, the Rxe2x80x2 group is ethyl or isopropyl. In another preferred embodiment, the R is unsaturated having at least one carbon-carbon double bond in it""s carbon chain. Preferably, the oleaginous fluid is from about 30% to about 99% by volume of the total of the fluid. In one illustrative and preferred embodiment, the oleaginous fluid is composed of from about about 5% to about 100% by volume of the oleaginous fluid of a material selected from diesel oil, mineral oil, synthetic oil, esters, ethers, acetals, di-alkylcarbonates, olefins, combinations of these and other similar fluids used in the formulation of well bore fluids.
Optionally, the illustrative fluid may further contain a non-oleaginous fluid. Such a non-oleaginous fluid preferably constitutes from about 1% to about 70% by volume of the total fluid. In one preferred and illustrative embodiment, the non-oleaginous fluid is selected from sea water, a brine containing organic or inorganic dissolved salts, a liquid containing water-miscible organic compounds, combinations of these and similar fluids used in the formulation of similar fluids. The illustrative fluid may also optionally include weighting or bridging agents. In one preferred and illustrative embodiment, the weighting or bridging agent is selected from calcium carbonate, dolomite, siderite, barite, celestite, iron oxides, manganese oxides, ulexite, carnalite, sodium chloride combinations of these and similar compounds used in the formulation of drilling and wellbore fluids.
In addition to the fluid compositions disclosed herein, the present invention also includes the use of such fluids in the formation, recompletion and maintenance of subterranean wells. Thus, one such illustrative embodiment includes a method of forming a subterranean well. The illustrative method includes drilling the subterranean well with a rotary drill bit and a drilling fluid that includes: an oleaginous based continuous phase and a solids tolerance agent as described above. Other methods of using the fluids disclosed are also contemplated as being part of the present invention. Such other methods and uses include using the above fluids as packing fluids, completion fluids, work-over fluids, fracturing fluids, spacer fluids and other such uses for well bore fluids that are known in the art.
These and other features of the present invention are more fully set forth in the following description of embodiments of the invention.